This invention relates to a radiation detector suitable for use, for example, in a positron CT (i.e. a positron computerized axial tomography) for measuring the distribution of a positron emitting nuclide administered to a human body.
The positron CT is a device for exhibiting the measuring results in images obtained by administering a positron emitting isotope (RI) such as .sup.11 C, .sup.13 N, .sup.15 O, .sup.18 F, or the like to a patient, detecting .gamma.-rays (511 KeV) emitted at opposite directions by two radiation detectors positioned around the patient simultaneously, and measuring the distribution of RI in the patient.
Heretofore, in order to improve a spatial resolution of the positron CT, it is proposed to use a multi-channel type radiation detector wherein each one single crystal scintillator of GSO (cerium doped gadolinium silicate Gd.sub.2 SiO.sub.5 :Ce) and one single crystal scintillator of BGO (bismuth germanate Bi.sub.4 Ge.sub.3 O.sub.12) are combined with each one photomultiplier tube [L. Eriksson et al: IEEE Trans. Nucl. Sci., vol 33, No. 1, pp 446-451 (1986)]. Since a luminescence decay time of BGO is 300 ns and that of GSO is 60 ns, it is possible to judge which scintillator, BGO or GSO, passes a radiation due to the difference in the luminescence decay time in such a multi-channel type radiation detector. But in such a case, the time resolution is poor, since it is determined by the BGO scintillator which has a longer luminescence decay time. Therefore, there is a problem in that the effect of using the GSO scintillator which has a very short luminescence decay time is a little.
In order to improve the time resolution of positron CT, it is necessary to employ a combination of two kinds of scintillators having a very short luminescence decay time and at the same time having different luminescence decay times. But no combination of scintillators having such properties has been found.
On the other hand, as a scintillator having a short luminescence decay time which can be changed with the concentration of an element doping material, it is known CaF.sub.2 doped with Ce [B. Jone, et al: Nucl. Instr. and Meth., vol. 143, p 487 (1977)]. But this scintillator cannot be used practically, since a light output is also changed with the amount of Ce doped. Further, since this scintillator is small in the absorption coefficient for X-rays or .gamma.-rays due to a small effective atomic number and a small density, the time resolution becomes worse when applied to a positron CT.